The image comparation isn't advanced enough to identify minor color
 changes when convert to grey scale. So use simpler images to make
sure we do not get false positives.

The goal of image comparation is only for make sure background is
placed at the figures.

Using Image Document, this commit includes tests to assert
the conversion outcome when convert svg to png. The main goal
is assert if the image backgronud (external and embeded cases)
is incorpoted into the PNG generated.

The image comparation is very basic level, but enough for this tests.

It exposes an error when convert one svg that uses url with http.